The present invention relates to a fire detection system in which a preset fire detection sensitivity pattern is produced from a receiver to change the fire detection sensitivity of a fire sensor in accordance with the time of day.
In general, the fire detection sensitivity of a conventional fire sensor is fixedly set at a given value in view of a given fire monitoring area, when the fire sensor is installed. Actually, the sensitivity is set to be slightly higher than the given value in order to secure a reliable detection of fire at the worst. For this reason, there is the possibility that non-fire alarms are produced. Particularly when someone is in a room, the non-fire alarm is likely to be issued because tobacco smoke or heat is necessarily produced. In this case, therefore, it is desirable to set the sensitivity of the sensor at a lower level. From this viewpoint, Japanese Laid-Open Patent Application Publication No. 20, 900/78 discloses a fire detection system which checks as to whether someone is in a room or not, by interlocking a checking portion with the operation of a door key or a lamp switch of the room, and sets the sensitivity of the sensor at a higher level when no person is in the room, so that an early detection of a fire is secured.
As mentioned above, however, the prior fire detection system changes the sensitivity only in accordance with a checked result as to whether the lamp is on or off, or whether someone is in the room or not. While this prior fire detection system has the likelihood of an earlier fire detection compared with a fire detection system in which the sensitivity is fixed, the prior fire detection system, although having such an advantage, has a problem in that the accuracy of detecting a room condition is relatively low and accordingly frequent erroneous fire alarms are likely to be issued adversely.
D. J. Rasbash has disclosed in his paper entitled "The role of fire detection systems in protection against fires", Proceedings of the Symposium on Automatic Fire Detection, London, March 8-10, 1972, a statistical consideration of fire occurrences during the time of day, i.e., the probability (P.sub.L) of fires in buildings (other than dwellings) becoming large during the time of day and the probability (P.sub.F) of a fatal fire occurring in multi-floor houses during the time of day. According to this study, P.sub.L and P.sub.F are higher by about three times during the night, particularly at dawn when people are hardly active than during the daytime when people are active.
The fire detection system as disclosed in Japanese Laid-Open Patent Application Publication No. 20,900/78 can not deal with the above-described state of fire occurrences, the probability of which varies in accordance with the time of day, and hence it still involves the problem of erroneous fire alarm issuance.